WIM sensors are sensors between one and several meters long that are installed in the road to measure the dynamic ground reaction forces of vehicles, in order to determine their weight. An essential element of the sensor is the aluminium hollow profile, in which a preloaded sensor package is installed. Mountings provided at the top and bottom ensure an optimal flow of force through the measuring element. The hollow profile comprises a tube part, which is responsible for initial tension and sealing the measuring element. A known hollow profile for such purposes is described for example in U.S. Pat. No. 5,461,924, which is hereby incorporated herein in its entirety for all purposes by this reference.
The measuring elements in the sensor packages are typically piezoelectric. However, other measuring principles, such as resistive, piezoresistive, capacitive, optical or magneto-elastic are also possible. In order to be able to measure using the entire length of the sensor, the measuring elements are arranged at regular intervals along the sensor length. They are spaced at distances of a few centimeters. A sensor package comprises steel receiving plates, one each at the top and bottom, between which an insulation and an electrode foil are arranged. The measuring elements are arranged between the electrode foil and the adjacent receiving plate. Steel receiving plates are necessary to guarantee the required strength and evenness of the quartz receiving. To ensure that measurements can be taken with the same accuracy and sensitivity over the entire length of the WIM sensors, all of the measuring elements must be exposed to precisely the same tension conditions in the hollow profile. This requires a high degree of accuracy not only in the manufacture of the hollow profiles, but also in the sensor packages, the thickness of which must be uniform over the entire length thereof, within a narrow tolerance.
The production of steel receiving plates in the desired length from one to several meters and the required thickness is extremely difficult. The required surface tolerances such as flatness, parallelism, thickness, surface roughness, etc., as the quartz receiving can only be obtained by employing a grinding process that is almost impossible for such lengths, and at all events involves huge effort. The receiving plates are prone to bending and twisting.
A WIM sensor consisting of a plurality of single modules that are aligned to each other, wherein each element includes a sensor that is read out separately, is known from U.S. Pat. No. 5,942,681, which is hereby incorporated herein in its entirety for all purposes by this reference. These individual modules are mounted on a bottom rail. It has been found that the construction is very complicated, because each element must be manufactured, tensioned and sealed individually. Since the WIM sensors are also exposed to environmental influences in the street, the seal loses its sealing quality after a certain time, leading to the failure of individual sensors.
A WIM sensor system with individual elements that have been inserted in a tube is described in U.S. Pat. No. 5,265,481, which is hereby incorporated herein in its entirety by this reference for all purposes. It has proven difficult to insert the sensor elements in the closed tube.